| Magnesium and its alloys have broad application prospects in the field of biomedical applications due to their high specific strength,elastic modulus similar to human bone,and good biocompatibility.However,magnesium alloys have relatively high degradation rates in the physiological environment,which is extremely unfavorable to the service safety of implantable devices.Therefore,how to prepare a low-cost and high-performance magnesium alloy with good mechanical properties and corrosion resistance is a hotspot issue in many industries.In this study,we try to add low-cost Zr elements to the as-cast Mg-1.0 Yb wt.%alloy that with good mechanical and corrosion resistance balance developed in the early stage,hoping to further modify and optimize the microstructure through the reasonable Zr content,and give full play to the performance potential of the new magnesium alloy.Specifically,four Mg-1.0 Yb-xZr（x=0,0.2,1.0,1.5 wt.%）alloys were prepared with free,trace amount,slightly excessive,and excessive Zr addition according to the solubility of Zr in Mg.Revealing the effects of Zr content on the shape and size of grains,size and distribution of the second phase of as-cast alloys,in vitro degradation mode,composition and evolution of the corrosion product layer,strength plasticity and fracture mode,hemolysis rate,and cell activity of the alloy by comprehensive microstructure characterization,mechanical properties and biocompatibility tests,and the specific research conclusions were summarized as follows:Optical microscopy（OM）and scanning electron microscopy（SEM）showed that the alloy grains gradually changed from typical coarse columnar to equiaxial fine grains with the increase of Zr content,accompanied by the emergence and coarsening of Zr-rich particles.This indicates that Zr has positive heterogeneous nucleation and grain refinement effects on the studied Mg-Yb alloy.Furthermore,electrochemical and immersion tests in a simulated body fluid（SBF）environment showed that the differentiated microstructure formed by the different Zr content greatly affected the corrosion behavior:when the trace amount（0.2 wt.%）Zr was added,the heterogeneous mixed crystal structure composed of coarse columnar and fine equiaxed crystals formed a large potential difference locally,accelerating local corrosion.When excess（1.5 wt.%）Zr was added,although the grain boundaries provided by high-density equiaxed fine grains provided a positive corrosion barrier effect to a certain extent,the coarse Zr-rich particles and the micro-couple corrosion between the matrix predominated,accelerating the local corrosion failure.Interestingly,after the addition of a slight excess（1.0 wt.%）of Zr,uniform equiaxed crystals and finely Zr-rich particles formed a dense,uniform and continuous corrosion products film,which effectively inhibited the development of local corrosion and finally formed a uniform corrosion mode.The corrosion mode is the same as that of the Zr-free alloy.The corrosion rate calculation shows that Mg-1.0 Yb-1.0 Zr alloy can maintain a low corrosion rate under different immersion times,and the corrosion rate is about～2.08 mm·year-1 after 30 days’immersion.Further tensile mechanical properties tests showed that the addition of Zr can significantly improve the strength of the alloys.Specifically,as the Zr content increases,the yield strength gradually increases from～29 MPa for Mg-1.0 Yb alloy to～35 MPa（0.2 Zr）,～53 MPa（1.0 Zr）,and～89 MPa（1.5 Zr）.The significant increase in tensile yield strength is mainly due to the equiaxed grain refinement and precipitation strengthening of Zr-rich particles caused by the increase in Zr addition.In terms of plasticity,when slightly excessive 1.0 wt.%Zr was added,the elongation doubled from～4.8%（Mg-1.0Yb）to～9.8%,while when the addition of Zr continued to increase to 1.5 wt.%,the elongation decreased to～7.6%,which may be caused by the stress concentration caused by the coarsening of Zr-rich particles.Further study of fracture analysis also verified the rationality of this conjecture.Subsequent in vitro cytotoxicity experiments show that the hemolysis rates of 1.0 wt.%and 1.5 wt.%Zr alloys are both less than 5%,and the cell viability is around 100%,showing good biocompatibility,especially for Mg-1.0 Yb-1.0Zr alloy.Due to the special microstructure characteristics obtained by adding appropriate Zr content,the alloy ultimately achieved further improvement in mechanical properties without loss of corrosion resistance.To reveal the evolution of corrosion resistance of Mg-1.0 Yb-1.0 Zr alloys in simulated body fluids,electrochemical and immersion experiments were further carried out under different immersion times.The results showed that with the prolongation of immersion time,the arc radius of capacitive impedance gradually increased,the impedance mode value increased,and the peak phase angle approached 90°,showing a trend of shifting from high frequency to low frequency,indicating a decrease in the electrochemical reaction rate.The calculation showed that the corrosion rate of the alloy is about 2.67 mm·year-1 after 5 days of immersion,which is more than three times lower than when the alloy was just immersed.Subsequent immersion experiments also proved that with the extension of immersion time,the integrity of the corrosion layer and the overall corrosion resistance of the alloy were significantly improved.The analysis of the evolution characteristics of alloy corrosion performance based on the double electric layer theory showed that the capacitance of the double electric layer decreased and the charge transfer resistance increased with the increase of immersion time,indicating that the inner protective corrosion product layer of the alloy thickened and the difficulty of charge transfer increased,ultimately leading to a decrease in the corrosion rate and an improvement in corrosion resistance of the alloy as the immersion time increases.In summary,Zr addition can significantly affect the microstructure,second phase size and its distribution,corrosion behavior,and mechanical properties of Mg-1.0 Yb as-cast alloys.Thanks to the addition of an appropriate amount of Zr,Mg-1.0 Yb-1.0 Zr alloys have achieved excellent corrosion resistance matched with good mechanical properties.Due to its good cost advantage,it can be used as an ideal alternative for medical implantable materials and has a good application prospect. |
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